The present invention relates to a new soluble mono-alkyl Stannoic Acid catalyst and its use in preparing high molecular weight polyester polymers. More particularly, the present invention relates to the discovery that the catalytic activity of a mono-alkyl Stannoic Acid of the type typically used in (trans) esterification reactions for preparing high molecular weight polyester polymers can be dramatically increased by pre-reacting the catalyst with an alkylene glycol.
U.S. Pat. No. 4,014,858 describes a first stage polyesterification for the preparation of polybutylene terephthalate which proceeds by reacting in the presence of an organotin catalyst having one organo to each tin linkage (1) a polycarboxylic acid compound (acid, anhydride and/or methyl ester) comprising terephthalic acid in a concentration of at least 50 equivalent percent of the carboxy equivalents and (2) a polyhydric alcohol component comprising from 75 to 100 equivalent percent 1,4-butanediol with the polyhydric alcohol being present in a concentration of from 1.1 to 4 hydroxy equivalents per carboxyl equivalent. The organotin catalyst can be a hydrocarbyl Stannoic Acid having the structure: ##STR1## wherein each R contains from 1 to 24 carbon atoms and can be alkyl or cycloalkyl. It is noted that since the organotin catalyst can be heated with 1,4-butanediol without the formation of tetrahydrofuran, the polyhydric alcohol component and catalyst can be preheated to the initial reaction temperature, i.e., 165.degree. C. to 170.degree. C., and the preheated polyhydric alcohol composition can then be introduced into the reactor with the other reactants.